How to Install a Transom Drain Bung Without Causing Rot

There's a cruel irony built into one of the most common fittings on any boat. The transom drain bung exists to protect your boat, letting water drain out from inside the hull so it doesn't sit and cause damage. But installed the wrong way, that same fitting becomes the exact thing it was meant to prevent: a direct path for water to seep into the transom core. On a cored transom, that slow, invisible leak can turn into long-term rot before you ever know it's happening.

Dan Lee knows that risk better than most. Based in the Cotswolds, UK, Dan is a wooden boat builder, designer, and educator who splits his time between custom builds, classic wooden boat restorations, and teaching the craft through his YouTube channel and online courses. In his latest video, he tackles the drain bung problem head-on during the restoration of the Stapley, one of several classic wooden boats he's brought back to life. Rather than just bedding a fitting and calling it done, Dan walks through a two-step sealing method built around TotalBoat High Performance Epoxy and Thixo thickened epoxy, designed to give the transom real, lasting protection instead of a single point of failure. Here's how he did it.

Starting With an Inspection, Not a Drill

Before any hole gets drilled, Dan inspects both the inside and outside of the transom to understand what he's working with. On the Stapley, that means locating two low points inside the hull where water naturally collects, since installing a drain bung anywhere else means the water it's supposed to drain simply won't reach it.

He also uses a helpful trick for positioning: because the hull planking extends slightly past the transom, he can reference that overhang from the outside to line up his hole with the low point on the inside. Measuring the fitting itself tells him exactly how far up from the hull planking the center of the hole needs to be, so the fitting seats flush against the hull once everything is drilled and installed.

Drilling and the Potted Screw Method

With his marks set, Dan drills partway through from inside the boat until the bit just breaks through to the outside, then finishes the hole from the outside with a hole saw. It's not just about getting a clean hole. Drilling from both directions also gives him a chance to check the condition of the transom core as he goes.

For the mounting screws, Dan uses a technique worth knowing if you're doing any hardware installation on a cored substrate: the potted screw method. Instead of drilling a hole sized exactly to the screw, he oversizes the hole for roughly three-quarters of the screw's length. That extra space gets backfilled with epoxy later, which surrounds the screw in a solid ring of epoxy along most of its length. The result is a stronger hold in the substrate and a screw that's fully protected from water intrusion, rather than sitting directly against bare, absorbent wood.

Sealing the Substrate With High Performance Epoxy

This is the step Dan calls the most important part of the whole process, and it's where High Performance Epoxy does its work. Any time you're cutting into a cored transom, that exposed wood is vulnerable, and sealing it thoroughly before any fitting goes in is what actually prevents rot down the line.

Working on a hot day, Dan reaches for the slow cure hardener to buy himself extra working time. Using a small mixing pot and a straw-style cleaning brush to reach fully into the holes, he applies three separate coats of epoxy to the inside of both the primary holes and the oversized screw holes, letting each coat soak in for 10 to 15 minutes before adding the next. The first coat disappears almost immediately into the thirsty wood. By the third coat, with the epoxy just starting to gel, he's left with a smooth, even film that fully seals every edge of exposed substrate.

That sealed layer functions as a backup system in its own right. Even if the final bedding compound around the fitting were ever to fail, the transom itself stays protected because the wood is no longer exposed.

Installing the Fitting With Thixo

Once the substrate is sealed and cured, Dan moves to Thixo for the potted screw holes and to bond the drain bung fitting in place. Thixo's pre-thickened, two-part cartridge format is built for exactly this kind of job. Loaded into a standard caulking gun with a mixing nozzle attached, it dispenses already mixed and already thickened, so there's no separate step to thicken resin and load a syringe by hand.

Before installing the fitting, Dan waxes it along with the screws using a mold release wax, a small detail that matters. He doesn't want the fitting bonded permanently to the epoxy. He wants a perfect impression of it. With the fitting waxed, he fills the potted screw holes with Thixo from the bottom up, sets the fitting into place, drives the screws home, and wipes away the excess with a cleanup wipe before it cures.

The wax trick pays off the next day, once the epoxy is fully cured. Dan removes the fitting and screws entirely, revealing a precisely cast impression of the fitting and threads sitting in solid, sealed epoxy. That impression means the fitting will register back into exactly the same spot, and the screw threads are now permanently protected inside cured epoxy rather than bare wood, even before the final bedding step.

Bedding the Fitting for the Long Haul

With the epoxy impression in place, Dan applies a thin layer of a flexible marine bedding compound before setting the fitting back in for good. Because the epoxy has already created a precise mold, he only needs a thin gasket layer to seal the surfaces where the fitting meets the transom. He's careful to let the screws re-register into the same threads they cured into, so the potted epoxy underneath doesn't get damaged by cross-threading, then tightens everything back down and cleans up the excess.

The result is a drain bung installed with genuine two-stage protection. The substrate itself is fully sealed with epoxy, independent of the fitting, and the fitting is bedded with a compound flexible enough to be removed later for repainting or maintenance without tearing up the surrounding finish.

Why This Method Matters

It's easy to treat a drain bung as a five-minute hardware install, and on a solid transom, it often can be. But on a cored transom, skipping the sealing step is exactly how small hardware penetrations turn into major structural repairs years down the road. Dan's approach, sealing the substrate with High Performance Epoxy before the fitting ever goes in, then using Thixo to pot the screws and set the fitting, builds in protection at every stage instead of relying on a single bead of sealant to keep water out.

If you're planning your own drain bung installation, or any hardware penetration into a cored substrate, this two-step method is one worth following. Check out Dan Lee Boatbuilding's full video for the complete walkthrough and follow along with more of his restoration and build projects.